2001 Sentra jerking problem

Guys,
I have a '01 Nissan Senta GXE 1.8L. The problem is that the car is shaking whenever it stops at traffic light or at any other places being idle for more than 30sec. It is giving me hell of a pain.

Showed to two mechanics. One cleared the carbons in hoses. But the problem persists.Upon calling they said this could be due to BAD IAC valve. Howver I don't trust 'em anymore.

A trip to the Nissan Service center promised 900 bucks at the cost of changing mass air flow sensor. However I find this part cheap online (~$150) than theirs ($532).

Any suggestions, what is the source of the problem? is it IAC valve or MAF sensor?

can i buy the parts online and get it fixed by a mechanic to save some money??

Thanks,

are you sure its a sensor and not motor mounts?x

was it a forward and backwards hard jerking motion? i know i disconnected the MAF in my 2oosx and it did that. it was so uncontrollable i had to turn the car off to stop it. if it is similar to that then its the MAF